Aromatic polyimides are categorized into the highest level of heat resistance among polymers, also have excellent mechanical, electric, and other characteristics, and thus have been used as materials in various fields including the aerospace and the electronics.
Common aromatic polyimides are unsuitable to use particularly for melt molding or as the matrix resin of fiber-reinforced composite materials due to the poor processability.
To use the aromatic polyimide as the matrix resin for fiber-reinforced composite materials, thermal addition reactive polyimides are typically used as follows: the polyimides still having a low molecular weight is impregnated into fibers; and then the polyimides is crosslinked and cured in the final process. PMR-15 is exemplified as one of the representative examples of polyimide resins previously developed for fiber-reinforced composite materials. PMR-15 is a thermal addition reactive imide resin with about 1 to 6 of the repeating unit which has a main chain formed by condensation reaction between benzophenonetetracarboxylic acid diester and diaminodiphenylmethane, and 5-norbornene-2,3-dicarboxylic anhydride monoester (known as nadic anhydride) as an end-capping agent (Non-Patent Document 1 and Non-Patent Document 2).
The nadic acid at each terminal of PMR-15 undergoes crosslinking reaction through ring-opening addition reaction in thermal curing process, and is considered to generate no volatile component during the crosslinking reaction. The thermally cured PMR-15 resin exhibits a high glass transition temperature, and thus has been used as the matrix resin for fiber-reinforced composite materials.
The production of a PMR-15 prepreg is carried out by a process of dissolving benzophenonetetracarboxylic acid diester, diaminodiphenylmethane, and 5-norbornene-2,3-dicarboxylic anhydride monoester as the raw materials in various alcohols to give a solution and then impregnating fibers into the solution. The fiber-reinforced composite material is produced by the hot press molding of the prepreg stacked in advance under a vacuum condition.
Furthermore, imide oligomers prepared by using 4-(2-phenylethynyl)phthalic anhydride as the end-capping agent are considered to give excellent moldability of a composite material and excellent balance between the heat resistance and the mechanical characteristics of a produced composite material, and are described in Patent Document 1, Patent Document 2, Patent Document 3, Non-Patent Document 3, and Non-Patent Document 4, for example.
Patent Document 1 discloses the terminally modified imide oligomers that have a logarithmic viscosity number of 0.05 to 1, are obtained by the reaction of 2,3,3′,4′-biphenyltetracarboxylic dianhydride having a bend; and non-planar structure, aromatic diamine compounds, and 4-(2-phenylethynyl)phthalic anhydride, and give cured products having good heat resistance and mechanical characteristics. Patent Document 1 also discloses that the invention can give a novel, highly practical terminally modified imide oligomer and a novel cured product of the terminally modified polyimide having good toughness and mechanical characteristics such as heat resistance, elastic modulus, tensile strength, and elongation, as the advantageous effects.
Another preparation process of a composite material has been studied as follows: an amic acid oligomer, which is the precursor of an imide oligomer, is dissolved in an organic solvent such as N-methyl-2-pyrrolidone; then the solution is infiltrated into fibers to give a semidried amic acid wet prepreg partially containing the solvent, as an intermediate; and a plurality of the prepregs are stacked and then heated and pressurized in a vacuum condition to yield a composite material.
In addition, Patent Document 2 discloses a terminally modified imide oligomer represented by General Formula (4) and a cured product of the oligomer.

(In the formula, R12, R13, and R14 are an aromatic diamine residue; R12 is a divalent aromatic diamine residue derived from 9,9-bis(4-(4-aminophenoxy)phenyl)fluorene; m and n satisfy relations of n≥0 in the case of R14═R12, n≥1 in the case of R14═R13, m≥0, 1≤m+n≤20, and 0≤n/(m+n)≤1; and repeating units are optionally arranged in a block sequence or a random sequence.)
Patent Document 2 describes that the invention can give a terminally modified imide oligomer capable of being dissolved in organic solvents such as N-methyl-2-pyrrolidone at a high concentration and can give a novel cured product of the terminally modified polyimide having good toughness and mechanical characteristics such as heat resistance, elastic modulus, tensile strength, and elongation, as the advantageous effects.
In Patent Document 3, the inventors of the present invention have found that terminally modified aromatic imide oligomers by 4-(2-phenylethynyl)phthalic anhydride synthesized from aromatic diamines including 2-phenyl-4,4′-diaminodiphenyl ether, 1,2,4,5-benzenetetracarboxylic acid show high solvent solubility, excellent melt flowability and moldability at a high temperature, and give thermally cured products exhibiting excellent heat resistance and high mechanical characteristics.
In addition, the preparation of the composite materials has been studied as follows: the terminally modified imide oligomer is dissolved at a high concentration to give an imide oligomer solution; then the solution is impregnated into carbon fibers to give a semidried imide wet prepreg partially containing the solvent, as an intermediate; and a plurality of the prepregs are stacked and then subjected to thermal curing to yield a composite material. The preparation of the imide wet prepreg is for the purpose of uniformly attaching the imide oligomer solution onto the surface of carbon fibers.
Patent Document 4 discloses a cured product, a prepreg, and a fiber-reinforced composite material that are obtained by heating a solution prepared from an aromatic tetracarboxylic acid containing a 2,3,3′,4′,-biphenyltetracarboxylic acid component, an aromatic diamine containing no oxygen atom in the molecule, and an end-capping agent having a phenylethynyl group. Patent Document 4 describes that the invention can give a polyimide cured product having excellent heat resistance and oxidation resistance, as the advantageous effects.